Theories of binocular vision originally imagined by Newton provided the foundation for subsequent investigations of the visual system by early anatomists and physiologists. These studies led to the widely accepted concept that degree of optic fiber decussation in the chiasm is inversely related to frontal orientation of the optical axes of the eyes (law of Newton-Müller-Gudden). A survey of 23 species from 11 mammalian orders demonstrates that, in contrast to other visual pathways, the retinohypothalamic projection does not obey this general principle. In further contradiction, an unexpected finding in primates is the predominance of ipsilateral, rather than contralateral, retinal input to the suprachiasmatic nucleus. This unusual organization underlines the functional and evolutionary specificities of this 'non-image forming' visual pathway.